ByDonald Kirk, CorrespondentMarch 25, 2012

Seoul, South Korea — President Barack Obama today warned North Korea not to go through with its plan to test a long-range missile next month but was clearly uncertain about what to do about it. His frustration was evident in the vagueness of his threat to hold back on 240,000 tons of food aid promised in a deal reached by US and North Korean envoys on Feb. 29.

Obama did not say specifically that the US would refuse to provide the food. Rather, he hinted, it would be “difficult to move forward with that package if they showed themselves unable to meet commitments even a month later.”

The view that launching the rocket would be “a clear violation,” as Mr. Obama put it, of the deal in which North Korea declared a moratorium on long-range missile and nuclear tests dominated his meeting with South Korea’s President Lee Myong-bak on the eve of a two-day nuclear security summit of leaders of more than 50 countries.

The summit is about nuclear terrorism and accidents, not the nuclear programs of either North Korea or Iran, which have cooperated with one another in exchanging components and technology. Nonetheless, worries about North Korea were sure to take up most of the time of the leaders outside the formal conference.

The most significant conversation may be that between Obama and China’s President Hu Jintao. Obama made clear he would press hard for Mr. Hu to get tough with North Korea about its plans to fire off the rocket around the time of massive celebrations on April 15 of the centennial of the birth of Kim Il-sung, grandfather of North Korea’s young “supreme leader” Kim Jong-un.

“Rewarding bad behavior, turning a blind eye, trying to paper over provocative acts obviously is not working,” said Obama. He believed China did not “want to see North Korea with a nuclear weapon,” but “the question is what the Chinese are doing” to get North Korea to adopt a different approach.

In that spirit, Obama showed his solidarity with President Lee after getting back from a quick visit to the demilitarized zone between the two countries. The visit to the DMZ came as reaffirmation of the US-Korean alliance as well as appreciation of the role of the 28,500 US troops still based in Korea.

After peering through binoculars into North Korea, Obama returned to this capital 40 miles below the North-South line armed with a warning of “the real consequence” if the North fires the rocket. North Korea, he said, “will have missed an opportunity ... to take a different path from the one they’re taking.”

North Korea insists it wants to launch the rocket solely to put a satellite into orbit, but experts here do not accept that claim since North Korea conducted similar tests in August 1998 and April 2009.

The widespread view is that Kim Jong-un, in his late 20s, sees next month’s test as a way to solidify his power. “The new leader has to stabilize his position by showing his capability,” says Kim Keung-koo, a nuclear physicist with the Korea Atomic Energy Research Institute.

Dr. Kim Chang-kyu, a nuclear scientist at the same institute, says North Korea “has the possibility” of firing a nuclear warhead if they can bring it down to a small size but “they are only at the beginning stage.”

Obama’s visit to the demilitarized zone dramatized a confrontation that has persisted since the Korean War ended with an armistice in July 1953.

Clad in a dark windbreaker, Obama looked across the DMZ from Observation Post Ouelette, named for an American soldier killed in the early months of the Korean War in 1950. North Korea, he observed, was “like a time warp, a country that missed 40 to 50 years of progress” and was unable do anything for the “well-being for its people.”

Just south of the DMZ, a 2.5-mile wide strip that stretches across the Korean peninsula, he chatted with US troops sharing duties with South Koreans in and around the truce village of Panmunjom on the North-South line.

“I could not be prouder of what you’re doing,” he told about 50 of them in the mess hall. “You guys are at freedom’s frontier.”

While he was there, reports circulated here that North Korea was already rolling its rocket by train to the launch site.

“The situation remains unsettled,” said Obama, asked what he thought of Kim Jong-un. “It’s not clear who’s calling the shots.” Whoever’s in charge, he surmised, “they have not yet made that strategic pivot where they say what they’re doing is not working.”